1. Field of the Invention
The present invention relates to the field of digital images, and more particularly to the field of watermarking digital images.
2. Description of Related Art
Watermarking has been in use for hundreds of years. Its earliest form was that of putting a physical impression into blank paper. Much later in the history of watermarking, the watermark was used as a way of authenticating a paper document, such as money, or other important legal documents [Ingemar Cox, Jeffrey Bloom, Matthew Miller, Digital Watermarking: Principles & Practice, 2001, Morgan Kauffman Publishers]. Our interest is in inserting a watermark into a digital image. This is a topic of recent interest. Due to the proliferation of electronic images on the Internet starting in the 1990s and the accompanying use of email, the owners or custodians of digital images wish to protect their intellectual property rights. Also, of interest is the ability to authenticate that a digital image is actually the digital image that one was intended to receive. The present invention relates to the field of authentication watermarks. The present invention is an authentication watermarking process of a digital image. Many of the existing watermarks of digital images are for proof of ownership. Authentication watermarks of digital images are an equally important area of watermarking.
A watermark is a semi-fragile watermark if it is robust to some degradation that might normally occur due to image processing (compression for example), while it is at the same time destroyed if the digital image is tampered with. Thus, an effective authentication watermarking method will work with JPEG images, which is a desired property. However, an authentication watermark should not be robust to gross image degradation due to tampering, which is also a desired property. Some watermarks are digital hashes of the digital image which are added onto the file. This is not a desired property because it changes and lengthens the original file format of the digital image.
In the area of Fourier optics, phase based filters, and in particular the binary phase only filter are used in image reconstruction [A. Oppenheim and J. Lim, “The importance of phase in signals,” Proc. IEEE, vol. 69, pp. 529-541, May 1981] and correlation based pattern recognition applications [J. L. Horner and J. R. Leger, “Pattern Recognition with Binary Phase-only Filters,” Applied Optics, Vol. 24, No. 5, pp. 609-611, 1985; David L. Flannery and Joseph L. Homer, “Fourier Optical Signal Processors”, Proc of the IEEE, vol. 77, no. 10, 1989]. These filters are obtained by transforming the digital image from the spatial domain to the frequency domain via a discrete Fourier transform. Once in the frequency domain the transformed image is mathematically manipulated to obtain the phase based filters [D. Psaltis, E. Pack, and S. Venkatesh, “Optical Image correlation with binary spatial light modulator,” Opt. Eng., pp. 698-704, 1984; J. L. Homer and P. D. Gianino, “Phase-only matched Filtering,” Appl. Opt., vol. 23, pp. 812-816, 1984].
No existing watermarking method uses a Fourier optics based approach. This is a deficiency in the field of watermarking because Fourier optics allows one to modify the original image without gross changes to the phase of the image. Thus, Fourier optics allows an unobtrusive watermark to be added to an image. A watermark that modifies an image too much is not desirable. Also, the field of Fourier optics is well developed for the use of image processing. Since watermarking an image is an aspect of image processing, using a Fourier optics based watermark is a natural extension of Fourier optics.